GDOI GROUPKEY-PUSH Acknowledgement Message
draft-weis-gdoi-rekey-ack-07

Versions: 00 01 02 03 04 05 06 07                        Standards Track
                                                        IPR declarations
Network Working Group                                            B. Weis
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                               U. Mangla
Expires: March 12, 2018                            Juniper Networks Inc.
                                                                 T. Karl
                                                        Deutsche Telekom
                                                           N. Maheshwari
                                                       September 8, 2017


               GDOI GROUPKEY-PUSH Acknowledgement Message
                      draft-weis-gdoi-rekey-ack-07

Abstract

   The Group Domain of Interpretation (GDOI) includes the ability for a
   Group Controller/Key Server (GCKS) to provide a set of current Group
   Member (GM) devices with additional security associations (e.g., to
   rekey expiring security associations).  This memo adds the ability of
   a GCKS to request the GM devices to return an acknowledgement of
   receipt of its rekey message, and specifies the acknowledgement
   method.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 12, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements notation . . . . . . . . . . . . . . . . . .   3
     1.2.  Acronyms and Abbreviations  . . . . . . . . . . . . . . .   4
   2.  Acknowledgement Message Request . . . . . . . . . . . . . . .   4
     2.1.  REKEY_ACK_KEK_SHA256 Type . . . . . . . . . . . . . . . .   5
     2.2.  REKEY_ACK_LKH_SHA256 Type . . . . . . . . . . . . . . . .   5
     2.3.  REKEY_ACK_KEK_SHA512 Type . . . . . . . . . . . . . . . .   5
     2.4.  REKEY_ACK_LKH_SHA512 Type . . . . . . . . . . . . . . . .   6
   3.  GROUPKEY-PUSH Acknowledgement Message . . . . . . . . . . . .   6
     3.1.  HDR . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  HASH  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  SEQ . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  ID  . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Group Member Operations . . . . . . . . . . . . . . . . . . .   8
   5.  GCKS Operations . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Management Considerations . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     7.1.  Protection of the GROUPKEY-PUSH ACK . . . . . . . . . . .  11
     7.2.  Transmitting a GROUPKEY-PUSH ACK  . . . . . . . . . . . .  12
     7.3.  Receiving a GROUPKEY-PUSH ACK . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The Group Domain of Interpretation (GDOI) [RFC6407] is a group key
   management method by which a Group Controller/Key Server (GCKS)
   distributes security associations (i.e., cryptographic policy and
   keying material) to a set of Group Member (GM) devices.  GDOI meets
   the requirement of the Multicast Security (MSEC) Group Key Management
   Architecture [RFC4046], and defines both a Registration Protocol and
   Rekey Protocol.  GDOI describes the Rekey Protocol as a GROUPKEY-PUSH
   message.





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   A GDOI GCKS uses a GROUPKEY-PUSH message (Section 4 of [RFC6407]) to
   alert group members to updates in policy for the group, including new
   policy and keying material, replacement policy and keying material,
   and indications of deleted policy and keying material.  Usually the
   GCKS does not require a notification that the group member actually
   received the policy.  However, in some cases it is beneficial for a
   GCKS to be told by each receiving GM that it received the rekey
   message and by implication has reacted to the policy contained
   within.  For example, a GCKS policy can use the acknowledgements to
   determine which GMs are receiving the current group policy and which
   GMs are no longer participating in the group.

   This memo introduces a method by which a GM returns an acknowledgment
   message to the GCKS.  Initially a GCKS requests GM to acknowledge
   GROUPKEY-PUSH messages as part of distributed group policy.  Then
   (shown in Figure 1) when the GCKS delivers a GROUPKEY-PUSH message,
   each GM that honors the GCKS request returns a GROUPKEY-PUSH
   Acknowledgement Message.  The rest of this memo describes this method
   in detail.

                GCKS                          GM1       GM2
                 |                             |         |
                 |                 +---------->|         |
                 |   GROUPKEY-PUSH |           |         |
                 |-----------------+           |         |
                 |                 |           |         |
                 |                 +-------------------->|
                 |                             |         |
                 |<----------------------------|         |
                 |      GROUPKEY-PUSH ACK      |         |
                 |                             |         |
                 |<--------------------------------------|
                 |      GROUPKEY-PUSH ACK      |         |



                    Figure 1: GROUPKEY-PUSH Rekey Event

   Implementation of the GROUPKEY-PUSH Acknowledgement Message is
   OPTIONAL.

1.1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].





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1.2.  Acronyms and Abbreviations

   The following acronyms and abbreviations are used throughout this
   document.

   D     Delete Payload

   GCKS  Group Controller/Key Server

   GDOI  Group Domain of Interpretation

   GM    Group Member

   HDR   Header Payload

   IV    Initialization Vector

   KD    Key Download Payload

   KDF   Key Derivation Function

   KEK   Key Encryption Key

   LKH   Logical Key Hierarchy

   MSEC  Multicast Security

   SA    Security Association

   SEQ   Sequence Number Payload

   SIG   Signature Payload

   SPI   Security Parameter Index

2.  Acknowledgement Message Request

   When a GM is ready to join a group, it contacts the GCKS with a
   GROUPKEY-PULL Registration Protocol.  When the GCKS has authenticated
   and verified that the GM is an authorized member of the group it
   downloads several sets of policy in a Security Association (SA)
   payload.  If the group includes the use of a GROUPKEY-PUSH Rekey
   Protocol, the SA payload includes an SA Key Encryption Key (KEK)
   payload (Section 5.3 of [RFC6407]).  When necessary the GROUPKEY-PUSH
   Rekey Protocol also contains an SA payload that includes SA KEK
   policy.  The SA KEK policy indicates how the GM will be receiving and
   handling the GROUPKEY-PUSH Rekey Protocol.




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   When the GCKS policy includes the use of the GROUPKEY-PUSH
   Acknowledgement Message, the GCKS reports this policy to the GM
   within the SA KEK policy.  The GCKS includes a new KEK Attribute with
   the name KEK_ACK_REQUESTED (value TBD-1), which indicates that the GM
   is requested to return a GROUPKEY-PUSH Acknowledgement Message.

   As part of the SA KEK policy, the GCKS specifies information on the
   keying material, that is used to protect the GROUPKEY-PUSH Rekey
   Protocol (e.g., presence of KEK Management Algorithm).  Parts of
   these information are used by a GM to derive the ack_key (defined in
   Section 3.2), which protects the GROUPKEY-PUSH Acknowledgement
   Message.  There are different types of Rekey Acknowledgement
   messages, which share an identical message format but differ in the
   used keying material.

   The following values of the KEK_ACK_REQUESTED attribute are defined
   in this memo.

2.1.  REKEY_ACK_KEK_SHA256 Type

   This type of Rekey ACK is used when the KEK KD Type (Section 5.6.2 of
   [RFC6407]) is part of the group policy.  The prf (defined in
   Section 3.2) is PRF-HMAC-SHA-256 [RFC4868].  The base_key (also
   defined in Section 3.2) is the KEK_ALGORITHM_KEY used to decrypt the
   GROUPKEY-PUSH message.  Note that for some algorithms the
   KEK_ALGORITHM_KEY will include an explicit Initialization Vector (IV)
   before the actual key (Section 5.6.2.1 of [RFC6407]), but it is not
   used in the definition of the base_key.

2.2.  REKEY_ACK_LKH_SHA256 Type

   This type of Rekey ACK can be used when the KEK_MANAGEMENT_ALGORITHM
   KEK attribute with a value representing Logical Key Hierarchy (LKH)
   is part of the group policy (Section 5.3.1.1 of [RFC6407]).  The prf
   is PRF-HMAC-SHA-256.  The base_key is the Key Data taken from the
   first LKH Key structure in an LKH_DOWNLOAD_ARRAY attribute (see
   Section 5.6.3.1 of [RFC6407]).  This is a secret symmetric key that
   the GCKS shares with the group member.  Note that for some algorithms
   the LKH Key structure will include an explicit IV before the actual
   key (Section 5.6.3.1 of [RFC6407]), but it is not used in the
   definition of the base_key.

2.3.  REKEY_ACK_KEK_SHA512 Type

   This type of Rekey ACK is identical to the REKEY_ACK_KEK_SHA256 Type,
   except that the prf is PRF-HMAC-SHA-512 (defined in [RFC4868]).





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2.4.  REKEY_ACK_LKH_SHA512 Type

   This type of Rekey ACK is identical to the REKEY_ACK_LKH_SHA256 Type,
   except that the prf is PRF-HMAC-SHA-512 (defined in [RFC4868]).

3.  GROUPKEY-PUSH Acknowledgement Message

   The GROUPKEY-PUSH message defined in [RFC6407] is reproduced in
   Figure 2.  The SA and Key Download (KD) payloads contain the actual
   policy and keying material being distributed to the GM.  The Sequence
   Number (SEQ) payload contains a sequence number that is used by the
   GM for replay protection.  This sequence number defines a unique
   rekey message delivered to that GM.  One or more Delete (D) payloads
   optionally specify the deletion of existing group policy.  The
   Signature (SIG) payload includes a signature of a hash of the entire
   GROUPKEY-PUSH message (excepting the SIG payload octets) before it
   has been encrypted

               GM                                    GCKS
               --                                    ----
                   <---- HDR*, SEQ, [D,] SA, KD, SIG

       * Protected by the Rekey SA KEK; encryption occurs after HDR

                   Figure 2: GROUPKEY-PUSH from RFC 6407

   When the GM has received a KEK_ACK_REQUESTED attribute in an SA KEK
   and it chooses to respond, it returns the value of the Sequence
   Number taken from the GROUPKEY-PUSH message to the GCKS along with
   its identity.  This tuple alerts the GCKS that the GM has received
   the GROUPKEY-PUSH message and implemented the policy contained
   therein.  The GROUPKEY-PUSH Acknowledgement Message is shown in
   Figure 3.

                      GM                              GCKS
                      --                              ----
                         HDR, HASH, SEQ, ID   ---->

              Figure 3: GROUPKEY-PUSH Acknowledgement Message

   The IP header for the GROUPKEY-PUSH Acknowledgement Message is
   constructed as if it were a reply to the GROUPKEY-PUSH message.  That
   is, the Source Address of the GROUPKEY-PUSH message becomes the
   Destination Address of the GROUPKEY-PUSH Acknowledgement Message and
   the GM includes its own IP address as the Source Address of the
   GROUPKEY-PUSH Acknowledgement Message.  The Source port in the
   GROUPKEY-PUSH message UDP header becomes the Destination port of the
   GROUPKEY-PUSH Acknowledgement Message UDP header, and the Destination



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   port of the GROUPKEY-PUSH message UDP header becomes the Source port
   of the GROUPKEY-PUSH Acknowledgement Message UDP header.

   The following sections describe the payloads in the GROUPKEY-PUSH
   Acknowledgement Message.

3.1.  HDR

   The message begins with a header as defined for the GDOI GROUPKEY-
   PUSH message in Section 4.1 of [RFC6407].  The fields in the HDR MUST
   be initialized as follows.  The Cookies of a GROUPKEY-PUSH message
   act as a Security Parameter Index (SPI) and are copied to the
   Acknowledgement Message.  Next Payload identifies a Hash payload
   (value 8) [ISAKMP-NP].  Major Version is 1 and Minor Version is 0.
   The Exchange Type has value 35 for the GDOI GROUPKEY-PUSH
   Acknowledgment Message.  Flags are set to 0.  Message ID MUST be set
   to zero.  Length is according to Section 4.1 of [RFC6407]).

3.2.  HASH

   The HASH payload is the same one used in the GDOI GROUPKEY-PULL
   exchange defined in Section 3.2 of [RFC6407].  The hash data in the
   HASH payload is created as follows:

        HASH = prf(ack_key, SEQ | ID)

   where:

   o  prf is specific to the KEK_ACK_REQUESTED value, and is described
      as part of that description.

   o  "|" indicates concatenation.

   o  SEQ and ID represent the bytes comprising the Sequence Number and
      Identification Payloads

   The ack_key is computed from a Key Derivation Function (KDF) that
   conforms to KDF in Feedback Mode as defined in NIST SP800-108
   [SP800-108] where the length of the derived keying material is the
   same as the output of the prf, there is no initialization vector, and
   the optional counter is not used.  Note: When the derived ack_key is
   smaller than the prf block size (i.e., 512 bits for PRF-HMAC-SHA-
   256), it is zero filled to the right, as specified in Section 2.1.2
   of [RFC4868].

        ack_key = prf(base_key, "GROUPKEY-PUSH ACK" | SPI | L)

   where:



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   o  prf is specific to the KEK_ACK_REQUESTED value, and is described
      as part of that description.

   o  base_key is specific to the KEK_ACK_REQUESTED value, and is
      described as part of that description.  If the base_key is smaller
      than the prf block size (i.e., 512 bits for PRF-HMAC-SHA-256),
      then it is zero filled to the right, as specified in Section 2.1.2
      of [RFC4868].

   o  "|" indicates concatenation.

   o  "GROUPKEY-PUSH ACK" is a label encoded as a null terminated ASCII
      string.

   o  SPI is the Initiator Cookie followed by the Responder Cookie taken
      from the GROUPKEY-PUSH message HDR, which describes the Context of
      the key usage.

   o  L is a length field matching the number of bits in the ack_key.  L
      MUST match the length of the base_key (i.e., 512 bits for PRF-
      HMAC-SHA-256).  The value L is represented as two octets in
      network byte order (that is, most significant byte first).

3.3.  SEQ

   The Sequence Number Payload is defined in [RFC6407].  The value in
   the GROUPKEY-PUSH SEQ payload is copied to the SEQ payload.

3.4.  ID

   The Identification payload is used as defined in Section 5.1 of
   [RFC6407].  The ID payload contains an ID Type of ID_IPV4_ADDR,
   ID_IPV6_ADDR, or ID_OID as defined for GDOI exchanges [RFC8052].
   Protocol ID and Port fields MUST be set to 0.  The address provided
   in the ID payload represents the IP address of the GM, and MUST match
   the source IP address used for the most recent GROUPKEY-PULL
   exchange.

4.  Group Member Operations

   When a GM receives an SA KEK payload (in a GROUPKEY-PULL exchange or
   GROUPKEY-PUSH message) including a KEK_ACK_REQUESTED attribute, it
   records in its group state some indication that it is expected to
   return a GROUPKEY-PUSH ACK message.  A GM recognizing the attribute
   MUST honor the KEK_ACK_REQUESTED attribute by returning
   Acknowledgments, because it can be expected that the GCKS is likely
   to take some policy-specific action regarding non-responsive GMs,
   including ceasing to deliver GROUPKEY-PUSH messages to it.



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   If a GM cannot respond with the requested type of Acknowledgement, it
   continues with protocol exchange and participates in the group.  In
   any case, if a GM stops receiving GROUPKEY-PUSH messages from a GCKS
   it will re-register before existing security associations expire, so
   omitting sending Acknowledgements should not be critical.

   When a GM receives a GROUPKEY-PUSH message that contains a
   KEK_ACK_REQUESTED attribute in the SA KEK payload, it processes the
   message according to RFC 6407.  When it concludes successful
   processing of the message, it formulates the GROUPKEY-PUSH ACK
   messages as described in Section 3 and delivers the message to the
   GCKS from which the GROUPKEY-PUSH message was received.  A GROUPKEY-
   PUSH ACK message is sent even if the GROUPKEY-PUSH message contains a
   Delete payload for the KEK used to protect the GROUPKEY-PUSH message.

5.  GCKS Operations

   When a GCKS policy includes requesting a GROUPKEY-PUSH ACK message
   from Group Members, it includes the KEK_ACK_REQUESTED attribute in
   the SA KEK payload.  It does this each time the SA KEK is delivered,
   in both GROUPKEY-PULL exchanges and GROUPKEY-PUSH messages.  The
   value of the KEK_ACK_REQUESTED attribute will depend upon the type SA
   KEK, as described in Section 2.

   When a GCKS receives a GROUPKEY-PUSH ACK message (identified by an
   Exchange type of GROUPKEY-PUSH-ACK), it first verifies that the group
   policy includes receiving GROUPKEY-PUSH ACK messages.  If not, the
   message is discarded.  GCKS implementations SHOULD keep a record
   (e.g., a hash value) of recently received GROUPKEY-PUSH
   Acknowledgment messages and reject duplicate messages prior to
   performing cryptographic operations.  This enables an early discard
   of the replayed messages.

   If the message is expected, the GCKS validates the format of the
   message, and verifies that the HASH has been properly constructed as
   described in Section 3.2.  If validation fails, the message is
   discarded.  The GCKS extracts the sequence number and identity of the
   GM from the SEQ and ID payloads respectively, and records the fact
   that the GM received the GROUPKEY-PUSH message represented by its
   serial number.

6.  Management Considerations

   The GCKS manages both group policy and group membership of a group.
   Group membership policy includes a strategy to ensure that rekey
   messages with current group policy reach all live group members.
   This is discussed briefly in Section 5.3 of the MSEC Group Key
   Management Architecture [RFC4046].  The GROUPKEY-PUSH Acknowledgement



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   message specified in this memo provides the GCKS an additional method
   to assess if a group member is live and has received the current
   group policy.  But it is possible for a rekey message or GROUPKEY-
   PUSH Acknowledgement message to be discarded in the network, which
   results in a live GM to appear unresponsive.  Also a GM might not be
   able to respond with an GROUPKEY-PUSH ACK.  So the GCKS should use
   caution in using a lack of Acknowledgment as the only factor in
   determining whether a GM is live.  In particular, a GCKS SHOULD NOT
   consider a GM to have left the group until it has received at least
   one ACK from the GM.

   Some management considerations determining how a Group Member handle
   Acknowledgement messages is as follows:

   o  A GM MUST respond with Acknowledgement messages when requested, as
      a GCKS can subsequently determine when a GM becomes unexpectedly
      non-responsive.

   o  A GM receiving GROUPKEY-PUSH message as a multicast message MAY
      introduce a jitter to the timing of its Acknowledgement message to
      help the GCKS better manage replies from group members.  A GM MUST
      NOT delay sending an Acknowledgment for more than 5 seconds. a
      GCKS SHOULD NOT declare an Acknowledgment as missing until it has
      waited at least 10 seconds.  Implementations SHOULD make these
      timers configurable.

   Some management considerations determining how the GCKS handles
   Acknowledgement messages is as follows:

   o  A non-receipt of an Acknowledgement is an indication that a GM is
      unable to respond.  A GCKS SHOULD wait at least several seconds
      before determining non-receipt, as GMs could add jitter to the
      response time before sending an acknowledgement.

   o  If the GCKS is aware that GMs are expected to respond, then a non-
      receipt of an Acknowledgement SHOULD trigger a logging event.  The
      GCKS MAY be configured with additional policy actions such as
      transmitting the GROUPKEY-PUSH message several times in a short
      period of time (as suggested in [RFC4046]), which mitigates a
      packet loss of either the GROUPKEY-PUSH message or an
      Acknowledgement message.  Another policy action could be to
      alerting GCKS administrators of GMs that do not return several
      consecutive acknowledgement messages or even removing unresponsive
      GMs from the group.  However, a GCKS with a policy of removing GMs
      from the group needs to be aware that a GM that has not responded
      will not receive newer group policy until it initiates contact
      with the GCKS again.




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   o  When a GROUPKEY-PUSH message includes a Delete payload for the KEK
      used to protect the GROUPKEY-PUSH message, the GCKS SHOULD NOT
      itself delete the KEK until it has given GMs the opportunity to
      acknowledge receipt of the GROUPKEY-PUSH message.  This could be
      several seconds, as GMs could add jitter to the response time
      before sending an acknowledgement.

   o  A GCKS SHOULD log failure events, such as receiving
      Acknowledgement messages for a group in which the GCKS has not
      requested Acknowledgements, receiving malformed Acknowledgement,
      and Acknowledgements that fail validation.

7.  Security Considerations

   There are three areas of security considerations to consider: the
   protection of the GROUPKEY-PUSH ACK message, whether the GM should
   transmit a GROUPKEY-PUSH ACK, and whether a GCKS should accept a
   GROUPKEY-PUSH ACK.  These are addressed in the following subsections.

   The construction of the HASH defined in this memo uses PRF-HMAC-
   SHA-256 or PRF-HMAC-SHA-512.  The strength of these PRFs were
   unquestioned at the time this memo was developed.  When a HASH
   construction is necessary using a different prf, a new
   KEK_ACK_REQUESTED value will be defined in a new specification.

7.1.  Protection of the GROUPKEY-PUSH ACK

   The GROUPKEY-PUSH ACK message is an ISAKMP [RFC2408] message.
   Message authentication and Man-in-the-Middle Attack Protection is
   provided by the inclusion of a HASH payload, which includes the
   output of an HMAC computation over the bytes of the message.

   When the value of REKEY_ACK_KEK is specified, because the KEK is a
   group secret impersonation of a victim GM by another authorized GM is
   possible.  However, security considerations of the impersonation are
   limited to a false claim that a victim GM has received a GROUPKEY-
   PUSH when the victim GM has in fact not received it (e.g., because an
   active attacker has discarded the GROUPKEY-PUSH).  If a GCKS policy
   includes sending retransmissions of the GROUPKEY-PUSH message to that
   victim GM, then the victim GM might not receive replacement security
   associations.  However, this adds no additional threats over a use
   case where the GROUPKEY-PUSH ACK is not deployed and GROUPKEY-PUSH
   messages are withheld from a victim GM by an active attacker.  These
   threats can be mitigated by using a value of REKEY_ACK_LKH, due to
   the use of a secret pairwise key shared between the GCKS and
   individual GM.





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   Confidentiality is not provided for the GROUPKEY-PUSH ACK message.
   The contents of the message can be observed by a passive attacker,
   which includes the hash value, the sequence number of in the
   GROUPKEY-PUSH message to which it is acknowledging receipt, and the
   identity of the GM.  Observation of a hash value or set of hash
   values will not compromise the hash key.  The identity of the GM is
   also available to the passive attacker as the source IP address of
   the packet.  The sequence number does reveal the sequence number that
   was included in the GROUPKEY-PUSH, which was previously not available
   to the attacker.  However, the attacker is assumed to not be in
   possession of the key used to encrypt the message, and thus cannot
   create a spoofed GROUPKEY-PUSH message.  Therefore, there is no
   direct value that the attacker derives from the knowledge of the
   sequence number.

7.2.  Transmitting a GROUPKEY-PUSH ACK

   A GM transmits an ACK only when the policy of the most recently
   received SA KEK includes a request by the GCKS for ACKs, and only is
   returned after processing the GROUPKEY-PUSH message according to
   Section 4.4 of [RFC6407].  In other words, the form of the GROUPKEY-
   PUSH message will have been validated, replay protection completed,
   and the digital signature verified as being genuine.  Therefore, the
   threats of a GM responding to a spoofed or resent GROUPKEY-PUSH
   message, and the possibility of the GM being used to propagate a
   Distributed Denial of Service (DDoS) attack on a GCKS are mitigated.
   For more information, see the security considerations of a GROUPKEY-
   PUSH message described in Section 7.3 of [RFC6407].

7.3.  Receiving a GROUPKEY-PUSH ACK

   A GCKS receiving ACK messages will follow the validation steps
   described in Section 5 before interpreting the contents of the
   message.  The GCKS will then be sure to operate only on messages that
   have been sent by an authorized GM.

   A GCKS SHOULD be prepared to receive GROUPKEY-PUSH ACK messages from
   each GM to which it was sent.  That is, needs to ensure it has
   sufficient resources (e.g., receive queue size) so that it does not
   unnecessarily drop ACK messages.  An GCKS should be aware that a
   large number of replayed or invalid GROUPKEY-PUSH messages could be
   addressed to it.  However, this is no worse a threat than if it
   received a large number of other types of replayed or invalid GDOI or
   other messages containing a HASH payload.

   How a GCKS processes the serial number and identity included in an
   ACK message is a matter of local policy and is outside the scope of
   this memo.



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8.  IANA Considerations

   The following additions are made to the GDOI Payloads [GDOI-REG]
   registry.

   A new attribute is added to the SA KEK Payload Values - KEK
   Attributes registry.  The ID Class name is KEK_ACK_REQUESTED with a
   value of TBD-1, and is a Basic attribute.

   A new registry defining values for KEK_ACK_REQUESTED is needed, and
   these values are shown in the following table.  The terms Reserved,
   Unassigned, and Private Use are to be applied as defined in
   [RFC8126].  The registration procedure is Specification Required.

                   Value            Type
                  -------           ----
                     0              Reserved
                     1              REKEY_ACK_KEK_SHA256
                     2              REKEY_ACK_LKH_SHA256
                     3              REKEY_ACK_KEK_SHA512
                     4              REKEY_ACK_LKH_SHA512
                    5-128           Unassigned
                  129-255           Private Use

   A new registry describing ISAKMP Exchange Types for GDOI is added to
   GDOI Payloads [GDOI-REG].  This registry defines DOI Specific Use
   values [ISAKMP-EXCH], which are Exchange type values used with the
   ISAKMP GDOI DOI.  Its name is "GDOI DOI Exchange Types".  The
   registration procedure is Specification Required.  The terms Known
   Unregistered Use and Unassigned are to be applied as defined in
   [RFC8126].

             Value                      Phase        Reference
             ----                       -----        ---------
             GROUPKEY-PULL               32           RFC 6407
             GROUPKEY-PUSH               33           RFC 6407
             Known Unregistered Use      34
             GROUPKEY-PUSH-ACK           35           RFC XXXX
             Unassigned                36-239

   [Note to RFC Editor: Please replace XXXX with the number of the RFC
   resulting from this memo, and delete this note.]

9.  Acknowledgements

   Mike Hamada, Adrian Farrel, and Yaron Sheffer provided many useful
   technical and editorial comments and suggestions for improvement.




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10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC4868]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-
              384, and HMAC-SHA-512 with IPsec", RFC 4868,
              DOI 10.17487/RFC4868, May 2007, <https://www.rfc-
              editor.org/info/rfc4868>.

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
              October 2011, <https://www.rfc-editor.org/info/rfc6407>.

   [RFC8052]  Weis, B., Seewald, M., and H. Falk, "Group Domain of
              Interpretation (GDOI) Protocol Support for IEC 62351
              Security Services", RFC 8052, DOI 10.17487/RFC8052, June
              2017, <https://www.rfc-editor.org/info/rfc8052>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

10.2.  Informative References

   [GDOI-REG]
              Internet Assigned Numbers Authority, "Group Domain of
              Interpretation (GDOI) Payload Type Values", IANA Registry,
              November 2016, <http://www.iana.org/assignments/gdoi-
              payloads/gdoi-payloads.xml>.

   [ISAKMP-EXCH]
              Internet Assigned Numbers Authority, "Internet Key
              Exchange (IKE) Attributes Exchange Type Values",
              IANA Registry, May 2013, <http://www.iana.org/assignments/
              ipsec-registry/ipsec-registry.xhtml#ipsec-registry-17>.

   [ISAKMP-NP]
              Internet Assigned Numbers Authority, "Internet Key
              Exchange (IKE) Attributes Next Protocol Types",
              IANA Registry, May 2013, <http://www.iana.org/assignments/
              ipsec-registry/ipsec-registry.xhtml#ipsec-registry-21>.




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   [RFC2408]  Maughan, D., Schertler, M., Schneider, M., and J. Turner,
              "Internet Security Association and Key Management Protocol
              (ISAKMP)", RFC 2408, DOI 10.17487/RFC2408, November 1998,
              <https://www.rfc-editor.org/info/rfc2408>.

   [RFC4046]  Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
              "Multicast Security (MSEC) Group Key Management
              Architecture", RFC 4046, DOI 10.17487/RFC4046, April 2005,
              <https://www.rfc-editor.org/info/rfc4046>.

   [SP800-108]
              Chen, L., "Recommendation for Key Derivation Using
              Pseudorandom Functions", United States of America,
              National Institute of Science and Technology, NIST Special
              Publication 800-108, October 2009,
              <http://dx.doi.org/10.6028/NIST.SP.800-108>.

Authors' Addresses

   Brian Weis
   Cisco Systems
   170 W. Tasman Drive
   San Jose, California  95134-1706
   USA

   Phone: +1-408-526-4796
   Email: bew@cisco.com


   Umesh Mangla
   Juniper Networks Inc.
   1133 Innovation Way
   Sunnyvale, California  94089
   USA

   Phone: +1-408-936-1022
   Email: umangla@juniper.net


   Thomas Karl
   Deutsche Telekom
   Landgrabenweg 151
   Bonn  53227
   Germany

   Phone: +49 228 18138122
   Email: thomas.karl@telekom.de




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   Nilesh Maheshwari

   Email: nileshm@gmail.com
















































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